1. Field of the Invention
The invention in general relates to apparatus for detecting incipient flaws in structures.
2. Description of the Prior Art
Acoustic emission signals are sometimes generated in structures due to stressing of the structural material as well as by crack initiation and/or growth. These acoustic emissions are high frequency elastic waves that travel through the material and are indicative of possible structural degradation.
Accordingly, acoustic emission instrumentation systems are provided for monitoring the structural integrity by the use of an array of acoustic emission detectors placed over a structure in conjunction with a triangulation system which uses information on the detector position coordinates, the stress wave arrival time at the detectors, and various parameters to compute the location of an acoustic emission source. Once located, the suspect zone of activity is pinpointed for later diagnostic inspection which may be done using high resolution ultrasonic imaging techniques or X-rays.
One type of transducer assembly which may be utilized as a passive listening device to detect acoustic emission is described in U.S. Pat. No. 3,935,484. The assembly utilizes a piezoelectric element which is coupled to an acoustic window of the assembly and is compressed against it. The transducer element is responsive to surface displacement in a normal direction caused by an acoustic emission.
Under certain circumstances, it would be desirable to provide a detector which not only was responsive to compressional wave energy but also shear wave energy so as to provide somewhat more information relative to the acoustic emission. One way of accomplishing this is to place additional shear wave detectors in the general area of the compressional wave detector. However, this arrangement adds to the complexity and total cost of the system. One type of measuring cell described in U.S. Pat. No. 3,566,163 is sensitive to both shear and compressional forces, and is packaged in a self-contained integral assembly. It is, however, used for force or acceleration measurements. The measuring cell utilizes six piezoelectric discs with interposed electrodes, and such arrangement if it were adapted to detect acoustic emissions would exhibit a relatively low efficiency due to the many couplings through the interfaces and electrodes, and consequent distortion of meaningful output signals.